NEW YORK — Henderson, a London-based investment firm, plans to combine with U.S. rival Janus Capital and create a new company that would manage more than $320 billion in assets.

The combined company will be called Janus Henderson Global Investors and its stock will be traded on the New York Stock Exchange when the deal closes, which is expected to happen in the second quarter of next year.

Henderson shareholders will own about 57 percent of the new company and Janus shareholders will own 43 percent.

Shares of Henderson Group PLC, traded on the London Stock Exchange, rose nearly 17 percent Monday. Shares of Denver-based Janus Capital Group Inc., traded on the NYSE, rose more than 13 percent.